It is known in the art to form a sensor array by providing an optical fiber with multiple sensing segments separated by weakly reflecting portions, such as fiber Bragg grating reflectors. The sensing segments undergo a change in refractive index in response to changes in a physical condition (such as temperature, pressure, refractive index value), resulting in changes in the transmitted optical signal. An analysis of the transmission spectrum of the output optical signal can then be used to determine these local changes in physical condition along the length of the fiber.
In particular, a fiber sensor array may be used to monitor the characteristics of a medium (e.g., gas or liquid) adjacent to a fiber as well as the fiber itself. The characteristics include, for example, changes in temperature, pressure, refractive index, electromagnetic field, mechanical properties (stress, strain), chemical properties (introduction of a noxious gas into the ambient, for example), etc., where the characteristics are monitored by measuring one or more variations in the properties of light propagating along the optical fiber. Suitable properties to be measured include, but are not limited to, spectrum, polarization, pulse characteristics and the like.
One conventional type of fiber sensor array utilizes a plurality of fiber Bragg gratings (FBGs) formed along disparate sections of an optical fiber. An FBG sensor array monitors shifts of narrow Bragg resonances in response to changes in the environment, where each grating forming the array has a different, narrow resonant wavelength. As a result, a plurality of FBGs with different resonant wavelengths may be disposed in series along a single optical fiber so that all of the individual resonance shifts are separated and can be determined from a single transmission or reflection spectral measurement.
While useful in forming an “array” sensor that requires an analysis of only a single output signal, the need to evaluate several different, narrow resonances along the spectrum requires expensive optical analyzers with the required fine spectral resolution capability. Additionally, FBG sensors have somewhat limited applicability inasmuch as the gratings themselves degrade at relatively high temperatures. In particular, UV-inscribed gratings become unstable at elevated temperatures. Thus, FBG sensor arrays are not suitable for applications where the sensor may be exposed to extreme temperature (or other environmental) conditions.
In contrast, other types of sensors, such as Mach-Zehnder fiber interferometer sensors, are more robust and can be used in high temperature environments, since they do not require include UV-inscribed gratings. A Mach-Zehnder interferometer (MZI) type of sensor does not use resonance wavelength analysis and is considered to be more broadband than an FBG sensor. Advantageously, since there is no need to monitor shifts in narrow wavelength resonances, the MZI sensor does not require the use of expensive optical spectrum analyzers. However, it is difficult to arrange the MZI type of sensor in an array configuration so that a plurality of individual sensors receive a common input signal and their output signals can thereafter be combined and applied as an input to a single optical spectrum analyzer or other type of detector arrangement. Unlike the spectral resonances of the FBG sensor array, the broadband element spectra of an MZI strongly overlap and it is unclear how to separate individual sensor contributions from the spectrum of the entire array.
Therefore, a need remains for a fiber sensor array configuration that retains the robust, broadband attributes of an MZI-based system, yet is able to provide the individual sensor-based results associated with an FBG array system and is also able to operate under high temperature conditions.